Hindsight bias is a tendency to overestimate the foreseeability of events that have actually happened. I.e., subjects given information about X, and asked to assign a probability that X will happen, assign much lower probabilities than subjects who are given the same information about X, are told that X actually happened, and asked to estimate the foreseeable probability of X. Experiments also show that instructing subjects to "avoid hindsight bias" has little or no effect.